This invention relates to adhesive compositions, and more particularly, to adhesive compositions cured by subjection to ionizing radiation which promotes crosslinking of elastomeric block copolymers contained in the composition during exposure to the radiation.
Curing of adhesives based on conjugated diolefins and, optionally, vinyl aromatics has increased the range of service properties for such adhesives. Radiation curing and chemical curing of polymers to make such adhesives is known. This curing causes covalent crosslinking of the polymerized conjugated diolefins which is evidenced by a gel content of the crosslinked polymer. Before crosslinking, the polymers are melt processable but after crosslinking, the gel cannot be processed as melts. Crosslinking therefore enhances solvent resistance and improves elevated temperature shear properties. Compositions can therefore be applied to a substrate in a melt and then crosslinked to form a superior adhesive.
U.S. Pat. Nos. 4,391,949 and 4,780,367 disclose star shaped block copolymers of vinyl aromatic hydrocarbons and conjugated dienes which are particularly useful in adhesive compositions. The common feature of these polymers is that they each contain arms which have at least one block of a vinyl aromatic hydrocarbon and at least one block of a conjugated diene and the arms are coupled at a central point with a coupling agent or coupling monomers or initiator. These polymers are especially amenable to radiation crosslinking such as described in U.S. Pat. No. 5,104,921. The polymers are combined with a tackifying resin and subjected to high energy ionizing radiation to initiate cure of the polymer and the tackifying resin.